Free-machining corrosion-resistant stainless steel

ABSTRACT

THIS INVENTION RELATES TO AN AUSTENITIC STAINLESS STEEL HAVING IMPROVED MACHINABILITY WITHOUT SACRIFICE IN CORROSION RESISTANCE, SUCH BEING ACHIEVED BY THE ADDITION OF SULFUR WITHIN A CRITICAL RANGE OF 0.04 TO 0.07 PERCENT.   D R A W I N G

Feb. 16, 1971 w, ov c EI'AL 3,563,729

FREE-MACHINING CORROSION'RESISTANT STAINLESS STEEL Filed AprillG, 1968 .2 Sheets-Sheet z l I ll 0 0.050 0. )0 0.15 0.20 0.32

Sulfur Content (wt PEA- I I I I I Q 1000- E 000 E I l l I I l l 0 0. 0s 0. /0 0.20 0.52

ARTHUR H S/(OW/TZ W Attorney United States Patent 3,563,729 FREE-MACHINING CORROSION-RESISTANT STAINLESS STEEL Curtis W. Kovach and Arthur Moskowitz, Pittsburgh, Pa., assignors to Crucible Inc., a corporation of Delaware Filed Apr. 16, 1968, Ser. No. 721,647 Int. Cl. C22c 39/20 US. Cl. 75-128 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an austenitic Stainless steel having improved machinability without sacrifice in corrosion resistance, such being achieved by the addition of sulfur within a critical range of 0.04 to 0.07 percent.

There is wide application for austenitic stainless steels for making parts, such as valves and fittings, that are used in corrosive environments. Machinablity is an important propery in these applications because of the great amount of machining involved in the manufacture of many of these parts, and resistance to corrosion is important from the standpoint of achieving long service life.

It is known that the machining characteristicts of austenitic stainless steels may be improved by providing a highsulfur content. For this purpose sulfur contents as high as about 0.40 percent have been used. Although machinability is effectively improved by these high sulfur contents, such is achieved at the expense of corrosion resistance. In an effort to provide the desired free-machining characteristics in austenitic stainless steels, without a corresponding lessening of the corrosion resistance, it has been proposed to lower the sulfur content and replace the same with elements, such as aluminum or phos- It is a further object of the invention to provide an austenitic stainless steel characterized by good machinability and corrosion resistance without requiring the addition of large amounts of sulfur.

It is also an object of the invention to provide a corrosion resistant austenitic stainless steel having good machinability by employing relatively low amounts of sulfur without the inclusion of substantial amounts of other elements known to benefit machinability.

A more specific object of the invention is to promote the machinability of an austenitic stainless steel, without impairing corrosion resistance, by the inclusion of sulfur in an amount within the range of .04 to .07 percent, in the absence of significant additions of machinability promoting elements, such as aluminum, phosphorus and copper.

These and other objects of the invention, as well as a complete understanding thereof, may be achieved from the following description and drawing in which:

FIG. 1 is a graph showing the improvement in machinability achieved by the practice of the present invention, as demonstrated by data relating to drill machinability, tool life and lathe power;

FIG. 2 is a graph showing the effect of sulfur on the corrosion resistance of steel in accordance with the present invention in a 10 percent sulfuric acid solution at 170 F.;

FIG. 3 is a graph showing the effect of sulfur on the corrosion resistance of steel in accordance with the present invention in boiling glacial acetic acid; and

FIG. 4 is a graph showing the effect of sulfur in stainless steel in accordance with the present invention on the pitting resistance of the steel in simulated seawater,

The composition of the austenitic stainless steel in accordance with the present invention is within the following limits:

Manganese, percent; Phosphorus, percent; Silicon, percent Nickel, percent Chromium, percent. Molybdenum, percent Copper, percent Colum brum.

Titanium Tantalum Zirconium Nitrogen, percent Sulfur, percent-.. Selenium, percent.

Iron, percent phorus, that in combination with the sulfur will promote machinability. Also, elements such as copper, selenium, tellurium, bismuth, lead and silver have been used in combination with low sulfur contents to achieve the desired result. The inclusion of these elements, although effective at least to a degree for the purpose of improving machinability, has inherent disadvantages. For example, many of them will have a seriously detrimental effect on hot workability. If elements such as. phosphorus and aluminum are used in substantial amounts, the cleanliness of the steel will be adversely affected. In addition, the inclusion of copper in an effective amount for the purpose of machinability, which may typically be within the range of 1.5 to 5 percent, substantially increases the cost of the alloy. The same is true for selenium and many of the other elements it used in substantial amounts.

It is accordingly the primary object of the present invention to provide an austenitic stainless steel characterized by a combination of machinability and corrosion resistance.

As will be demonstrated in greater detail hereinafter, austenitic stainless steels within the above-stated composition limits are characterized by improved machinability and good corrosion resistance. By maintaining sulfur within the relatively low range of .04 to .07 percent, which range is critical for the purpose of the invention, an unexpected improvement in machinability is achieved in combination with corrosion resistance much better than that resulting if greater amounts of sulfur are used in accordance with conventional practice for producing free-machining steel. Although conventional machinability-promoting elements, such as phosphorus, copper and selenium, may be used in small or residual amounts without detrimentally affecting the steel, such are not required, as will be demonstrated hereinafter, to achieve the desired result of obtaining an austenitic stainless steel characterized by a combination of good machinability and corrosion resistance.

For the purpose of demonstrating the invention, stainless steel compositions listed in Table I were melted. The

machinability data for these steels are also listed in mine the weight loss as a measure of corrosion. As may Table I. be seen from FIG. 2, by malntainmg the sulfur content at TABLE I Drill Lathe Lathe tool life Weight percent Hardmachinpower I mess ability require- 600 500 Bar Mn P S S1 N1 Cr M0 Cu Se (13 TIN) rating ment r.p.m. r.p.m.

* Number of cuts to failure.

The compositions listed in Table I were melted in a -pound air-induction furnace and cast into iron molds without aluminum deoxidation. The critical effect of sulfur, in accordance with the present invention, was demonstrated by sulfur additions ranging in amounts from 0.007 to 0.32 percent sulfur. All ingots were forged to 1 75 inch bars at temperatures between 1800 to 2100 F. Portions of the bars were turned to one-inch diameter rounds for machinability testing, and the remainder was hot-rolled to 0.100-inch sheet for corrosion testing. All machinability and corrosion tests, as reported hereinafter, were conducted on material in the solution-annealed condition, obtained by soaking one hour at 1950 F. and then water quenching. The results of the machinability tests are reported in FIG. 1.

The drill-machinability tests were conducted by comparing all bars to a standard AISI Type 303 test bar. The drill-machinability ratings were obtained by comparing the average drilling times for the test bars to the average drilling times for the standard Type 303 bar. Lathe tool-life tests were conducted by determining the number of cuts until tool failure. For this purpose a A- inch wide M-2 cutoff tool, ground with seven degree toprake and front-clearance angles, three degrees side clearance, and no front-relief angle, was used. The tests were conducted at a spindle speed of 600 r.p.m. A feed rate of 0.002 i.p.r. and a sulfurized oil lubricant were used. The lathe-power tests were conducted with a carbide tool that was used to make a standard plunge cut. In this test, the criterion for machinability is the power required to make the cut as measured by a wattmeter on the lathe motor. Low power indicates good machinability.

As may be seen from a study of FIG. 1, within the range of .04 to .07 percent sulfur, improved machinability is obtained. Above about .07 percent sulfur the increase in machinability becomes substantially less, particularly as demonstrated by the tool-life and lathe-power data, with respect to the amount of sulfur contained in the steel.

The specimens for corrosion testing, as reported in FIGS. 2 through 4, were prepared by belt grinding the hotrolled sheet to 0.080-inch thickness. Specimens 1 by 2 inches were then solution annealed and finished ground to 0.060 inch with dry No. 120 grit silicon carbide paper.

Corrosion tests, as reported in FIG. 2, were conducted by placing specimens as described above in a 10percent sulfuric acid solution maintained at a temperature of 170 F. The samples were maintained in the acid solution for a four-hour period and an eight-hour period. Upon removal from the acid, samples were weighed to deter- .07 percent or less, significant increased weight loss is avoided.

FIG. 3 similarly shows the effect of sulfur on corrosion resistance in the presence of boiling glacial acetic acid. Two test periods of 48 hours and 96 hours, during which the specimens were maintained in the boiling acid, were employed. As was the case with the sulfuric-acid test reported in FIG. 2, the samples subjected to the acetic acid did not show significant weight loss as long as sulfur was maintained at .07 percent or less. As may be seen from FIG. 3, when sulfur is increased beyond the upper limit of .07 perecent, weight loss is drastically increased.

Tests were also conducted to determine the effect of sulfur on the pitting resistance of the austenitic stainless steel. These tests were conducted in a simulated seawater environment prepared in accordance with the Navy Department Specification 44T27b, July 1, 1940. The specimens were exposed for eigtheen days at a temperature of 86 F. The pitting frequency was evaluated by visual counting at a magnification of 15X. The results of this test are shown in FIG. 4. As may be seen from this figure, at sulfur contents of .04 to .07 percent, which is within the range of the present invention, the pitting is relatively low.

From a comparison of the machinability test results with the corrosion test results, it may be seen that a hitherto unobtainable combination of machinability and corrosion resistance is achieved by closely controlling the sulfur content within the limits of .04 to .07 percent. This result is achieved by maintaining the other typical machinability-improving elements at substantially low levels, if present at all. In this manner problems, such as poor hot workability and lack of steel cleanliness, are avoided by avoiding the inclusion of elements promoting these effects while maintaining good corrosion resistance and im- :proving machinability.

Although various embodiments of the invention have been shown and described herein, it is obvious that other adaptations and modifications may be made by those skilled in the art within the sope and spirit of the appended claims.

What is claimed is:

1. An austenitic stainless steel characterized by a combination of machinability and corrosion resistance consisting esentially of, in percent, carbon .25 max., manganese 3 max., phosphorus .06 max., silicon 3 max., nickel 6 to 22, chromium 16 to 26, molybdenum 4 max., copper 1 max., up to 2 of at least one element selected from the group consisting of columbium, titanium, tantalum and zirconium, nitrogen .25 max., sulfur .04 to .07, and the balance iron and incidental impurities.

2. An austenitic stainless steel characterized by a combination of machinability and corrosion resistance consisting essentially of, in percent, carbon .15 max., manganese 3 max., phosphorus .06 max., silicon 2 max., nickel 6 to 14, chromium 16 to 20, molybdenum 4 max, cooper 1 max, sulfur .04 to .07, and the balance iron and incidental impurities.

3. The steel of claim 2 further restricted by carbon .15 max., nickel 10 to 14, chromium 16 to 19 and molybdenum 1.5 to 4.

4. The steel of claim 2 further restricted by carbon .15 max., nickel 8 to 12, chromium 16 to 19 and molybdenum linax.

References Cited UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner 

